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52 lines
2.1 KiB
Markdown
52 lines
2.1 KiB
Markdown
---
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title: "linker"
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aliases: Linker, linking
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tags:
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- cosc202
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---
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- Linkers facilitate use of code in multiple languages, e.g.
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- use C for programming speed of developer;
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- use Assembly language for running speed of code
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- But these days: often hard to beat optimising compilers!
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- Linkers effect interaction with libraries in two ways:
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- Static linking: build libraries’ code into your application
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- Dynamic linking: library connected in at load or run-time
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- Linkers facilitate interaction with OS
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## Static linking
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- Statically linked libraries are built into executable
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- We’ve previously discussed the dependency problems:
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- Every executable carries the weight of the library code
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- Also, bug fixing requires rebuilding the whole program
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- However, static linking is straightforward
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- e.g., nothing much OS-specific about static linking
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- Library calls are just function calls within userspace
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- Alternative is for linking not to finish at compile-time
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- Linker could add code to link at runtime
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- More commonly, linker tells OS more linking needed later
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## Dynamic linking
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- Dynamic linking is common on modern OSs, e.g.,
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- DLLs on Windows: Dynamic Link Libraries
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- SOs on Linux, Solaris: Shared Objects
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- DyLibs on macOS: Dynamically loaded modules
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- One copy of dynamic library shared by all executables
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- efficiently uses memory: load into RAM once; share to all
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- Fixing bugs / security flaws is streamlined:
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- just upgrade the single copy of the library
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- all processes will adopt fixed library (when they next start)
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## Managing dynamic libraries
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- Dynamic libraries may have different ownership
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- Some dynamic libraries will be part of the OS distribution
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- Others installed along with apps that need those libraries
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- Can track installable library use with reference counters
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- Software installer checks whether library is already present:
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- if not, install library, set reference count to one
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- if so, increase reference count by one
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- Uninstaller decreases reference count: deletes if at zero
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- But your reference counting needs to be reliable
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- ... unlike early Windows OSs: so called ‘DLL hell’
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